The invention relates to a method and an apparatus for controlling the mixture supplied to an internal combustion engine. The method and apparatus includes a provision for supplying supplementary combustion air to the engine under certain conditions.
In known methods for fuel control, the desired mixture composition is obtained by metering out to the aspirated air a predetermined quantity of fuel. An advantage of this method is that the fuel metering is easily performed because it is substantially unaffected by atmospheric pressure and temperature, at least to a lesser degree than would be the case for the metering of air. When internal combustion engines operate at exact values of the air-fuel ratio .lambda. and are run lean, i.e., with the air number .lambda..gtoreq.1, the torque depends heavily on the supplied fuel. Thus, while lean operation of the engine where .lambda..gtoreq.1 is desirable for several reasons, i.e., low fuel consumption and clean exhaust gas composition, there is an increased problem of torque fluctuations especially when the engine fluctuations are used as the control parameter in the control loop.